Myeloperoxidase, hydrogen peroxide and chloride ion form an antimicrobial system within phagocytic vesicles of blood monocytes and neutrophilic polymorphonuclear leukocytes. In studies using purified myeloperoxidase, antimicrobial activity was found to be modulated by nitrogenous compounds including amines, amides and ammonia. The premise of these studies is that myeloperoxidase-catalyzed oxidation of chloride ion within phagocytic vesicles of leukocytes results in formation of chloramine and/or chloramide derivatives of leukocyte components within the vesicle, as well as of components of phagocytized microorganisms. These derivatives containing nitrogen-chlorine (N-Cl) bonds are oxidizing and chlorinating agents with antimicrobial activity. The formation of membrane-permeable N-Cl derivatives may increase the rate of antimicrobial action, but may also result in diffusion of these agents out of the phagocytic vesicle, such that the leukocytes and host tissues are damaged. Formation of membrane-impermeable N-Cl derivatives will slow the rate of killing of microorganisms, but will also sequester the oxidizing equivalents within the phagocytic vesicle. Studies described in this proposal will characterize the interaction of the myeloperoxidase system with the nitrogenous components of leukocyte granules. The major class of such substances that are known to be present in the granules are the arginine-rich cationic proteins and other proteins. Other nitrogenous compounds that may be extracted from the granules will also be studied. The aim will be to characterize the chemistry of the interaction, and to study the effect of this interaction on antimicrobial activity. These studies will provide an understanding of the role of leukocyte components in regulating antimicrobial activity, and in protecting leukocytes and host tissues against damage by the myeloperoxidase system.